The invention relates to a window transparent to electron rays as well as to a method of manufacturing such a window, wherein said window comprises a foil which is transparent to electron rays and an element for supporting a peripheral region of the foil which is transparent to electron rays in the operational state. The invention also relates to an X-ray radiation device.
Such windows are used wherever sensitive objects are to be screened from external circumstances, while nevertheless a sufficient transparency for the passage of the electron ray is safeguarded. DE 198 21 939 A1 proposes the use of such windows in an X-ray tube with a liquid metal target, which is also referred to as LIMAX X-ray tube (LIMAX=LIquid Metal Anode X-ray tube). Such an X-ray device basically consists of an electron source and a target made of a metal which circulates in the operational state of the radiator. The liquid metal is present in a pump circulation system and is pumped from a divider head via a special steel plate into a receptacle. The electron ray hits the liquid metal flowing over the special steel plate and generates X-radiation therein. It is achieved by means of the window that the vacuum space of the electron source and the target are separated from one another so as to form two independent spaces, such that the target becomes less sensitive to the kind of flow and to the choice of liquid metal. A window used here comprises, for example, a diamond foil which is vapor-deposited on a silicon carrier substrate, whereupon the carrier substrate is partly removed for creating a window region or transmission zone for the electron ray. The window thus constructed is directly mounted in the X-ray tube.
It should be noted here that a distinction is made between the terms carrier substrate and retaining element in the context of the present invention. The carrier substrate serves as a deposition surface or auxiliary surface for manufacturing the window foil, whereas the retaining element serves as a positioning aid for the foil in its operational position.
It was found that windows known from DE 198 21 939 A1 are not resistant to pressure differences of more than 4 bar because at higher pressure differences the diamond film is torn from the silicon substrate owing to insufficient adhesion, i.e. the window bursts open. The bursting pressure is reached during the starting phase of X-ray tube operation, when pressure differences of more than 4 bar occur, in particular in the case of LIMAX tubes.
The invention accordingly has for its object to provide a window transparent to electron rays and a corresponding method of manufacturing such a window, which can remain reliably intact as a separation element under various conditions and/or fluctuating conditions between two spaces. In particular, a window is to be provided for overpressure and vacuum applications which is capable of withstanding pressure differences also of more than 4 bar in its operational state.
This object is achieved by means of a window transparent to electron rays which comprises a foil transparent to electron rays and separated from a carrier substrate as well as a retaining element for supporting a peripheral region of the foil transparent to electron rays in the operational state, wherein the retaining element is made of a material which has a linear thermal expansion coefficient adapted to the linear thermal expansion coefficient of the foil material, such that it is equal or similar thereto.
Preferably, the foil transparent to electron rays is made of diamond with a thickness of no more than 10 xcexcm. In an alternative embodiment, the foil may also be made of molybdenum or of beryllium.
It is preferable in the case of diamond foil that the retaining element is made of a material having a linear thermal expansion coefficient smaller than or equal to 9xc3x9710xe2x88x926/K; particularly preferred is the choice of a material having a linear thermal expansion coefficient lying within the range of 0.5-1xc3x9710xe2x88x926/K to 9xc3x9710xe2x88x926/K. The lower limit value follows from the linear thermal expansion coefficient of diamond. The linear thermal expansion coefficient of ideal diamond as a monocrystal lies at 0.5xc3x9710xe2x88x926/K, which coefficient rises to a value of approximately 1xc3x9710xe2x88x926/K in the manufacture by a CVD method and the accompanying formation of polycrystalline material.
The retaining element is preferably made of materials such as molybdenum with a linear thermal expansion coefficient between 5 and 6xc3x9710xe2x88x926/K, tungsten, titanium, tantalum, as well as their low alloys, glasses, ceramic materials with suitably low linear thermal expansion coefficients, also diamond, and possibly materials having a lower linear thermal expansion coefficient than diamond, especially than diamond in its polycrystalline form.
In a first advantageous embodiment, the foil transparent to electron rays and the retaining element are integrally made of diamond. Particularly advantageous here is the integral embodiment of the window with the retaining element, manufactured from an integral diamond plate with an original thickness of more than 10 xcexcm.
In a second, alternative embodiment, the foil transparent to electron rays and the retaining element are constructed as two parts, the foil with a thickness of less than 10 xcexcm, preferably less than 5 xcexcm, being provided on the retaining element with an interposed connecting layer. Both the foil and the retaining element may preferably each be made of diamond or each be made of molybdenum also in this second embodiment. Choosing the same material for the foil and the retaining element gives an optimum matching of the thermal expansion behaviors.
In contrast to a conventional window, which is formed by a carrier substrate with a foil deposited thereon and which does not withstand higher pressure differences because of the comparatively small adhesive forces between the carrier substrate and the foil, leading to a stripping of the foil from the carrier substrate, the window proposed here has a reliable connecting layer. The material of the retaining element is chosen such that its material behavior is adapted to that of the diamond foil, so that the two materials react to external influences with similar changes in volume. Overall, a window is obtained which withstands pressure differences of more than 4 bar and which is also suitable as a separation means for spaces in which different conditions prevail, for example because of differences in contents (liquids of different compositions in different aggregation states).
The connecting layer of the embodiment in two parts is preferably formed by a fusion layer of an active metal solder or a glass fusion. This is provided on the connecting surfaces of the retaining element. The carbide formers contained in the active metal solder such as, for example, titanium or molybdenum, react with the foil at the contact surfacexe2x80x94with the carbon present therein in the case of a diamond foilxe2x80x94so as to form metal carbides which achieve a fixed connection between the foil and the retaining element. Similarly advantageous is an adhesive layer, for example on the basis of an epoxy resin or a temperature-resistant ceramic adhesive, for example supplied by the Aremco Company. Preferably, the connecting layer may also be formed by a combined adhesion/fusion layer, in which case in particular the combination of glass fusion with ceramic adhesives is to be mentioned.
It is furthermore proposed that at least one surface of the foil transparent to radiation comprises at least one thickeningxe2x80x94extending to beyond the surface area of the foilxe2x80x94whose thickness amounts to at least 10% of the foil thickness. The proposed thickenings, representing mechanical reinforcement ridges or reinforcement patterns, should preferably, but need not necessarily, be of a thickness which is in particular smaller than the total thickness of the foil, but should be at least 10% of the foil thickness. The thickenings are provided at regular intervalsxe2x80x94for example as reinforcement elements running in parallel or forming a gridxe2x80x94or alternatively at irregular intervals. Said thickenings stabilize the foil mechanically while nevertheless leaving open regions of higher transparency for the electron ray.
Reference should be made here to EP 0 476 827 A1 which discloses windows which are transparent to X-rays, and which are thus of a different kind, because windows transparent to electron rays have to comply with fundamentally different boundary conditions for the transparency than X-ray-transparent windows. An X-ray window is described in this cited document which comprises a diamond foil transparent to X-rays, a carrier substrate, for example made of silicon, on which the diamond foil is deposited, and a carrier ring acting as a retaining element for supporting a peripheral region of the foil transparent to X-rays. The diamond foil is provided with reinforcement crosspieces also made of diamond on its surface for enhancing its mechanical strength. The carrier ring is made of aluminum. To manufacture such a window, a planar carrier substrate is vaporized with a gas containing carbon in a gas phase deposition processxe2x80x94for example a CVD (Chemical Vapor Deposition) processxe2x80x94, such that a diamond foil with a thickness of between 0.05 and 10 xcexcm is grown. A mask is provided which has recesses in those locations where the reinforcement ridges should lie, and which counteracts a diamond deposition in other locations. When the thickness of the reinforcement crosspieces has become greater than that of the foil, the deposition is stopped, the mask is removed, the carrier substrate is etched away centrally in the subsequent window region, and the substrate is connected to the carrier ring. The substrate may alternatively be fully etched away, in which case the aluminum carrier ring is directly connected to the diamond foil.
A manufacturing method for the integral embodiment according to the invention is proposed in which in a first step a monocrystalline or polycrystalline diamond plate with a thickness of between 10 and 1000 xcexcm is manufactured, which plate is thinned to a thickness transparent to an electron ray in a central region over a surface area corresponding at least to the diameter of the electron ray. This thinning is preferably achieved by means of a known laser or ion irradiation process. Depending on the diameter of the electron ray, this zone will typically have rectangular dimensions smaller than 5 to 2 mm. In an advantageous modification of the process, this integral window may be provided with reinforcement elements in that the central zone of the plate is irregularly thinned. It is advisable in this case to thin the edge regions of the central transmission zone less strongly, such that the thickened portions are present in the outermost regions of the thinned, i.e. processed zone. The passage of the electron ray through the transmission zone thus remains substantially unhampered. Thinning with different processing depths is controlled by means of the supplied power.
In an advantageous embodiment, moreover, electrically conductive diamond is to be used, which is achieved, for example, through doping of the diamond foil or the diamond plate with boron during the gas phase deposition.
Advantageously, the proposed window is used in an X-ray device having the characteristics defined in claim 16, but its use is obviously not limited to this application.